expected: |
  1,a,1,xxx
  2,b,,
query: |
  select t1.num, t1.name, t3.num, t3.value
  from 'Table1.csv' as t1
  left join (
    select * from 'Table2.csv' as t2 where t2.num > 8 and exists(select t4.value from 'Table2.csv' as t4)
      union select * from 'Table2.csv'
  ) as t3 on t1.num = t3.num
  limit 2;
